Ion trap



M. D. HARSH Oct. 15, 1957 10N TRAP Filed' March s1, 1954 N am wm V L wwNN NN QN Mm.

TTORNEY United States Patent O ION TRAP Maurice D. Harsh, Nefsville,Pa., assignor to-Radio Corporation of America, a corporation of DeiawareApplication March 31, 1954, Serial No. 419,965

6 Claims. (Cl. 315-8) This invention relates to a device forre-centering the electron beam in a cathode-ray tube after the entirebeam has been 'deflected to trap the negative ions in the beam.

In cathode-ray tubes using electromagnetic deflection, the negative ionspresent in the beam are essentially unaffected by the magneticdeflection field. These negative ions maintain an unwavering bombardmentof the fluorescent screen in one small area, thus causing a permanentdarkening or discoloration on the screen called an ion spot. These ionspots may be eliminated by an ion trap which consists of anelectrostatic lens field tilted to the longitudinal axis of the electrongun, and two magnetic fields that are transverse to the samelongitudinal axis. Th-e electrostatic lens field deflects the entirebeam from the axis of the electron gun. One magnetic field defiects theelectron portion of the beam back toward the axis of the cathode-raytube. The second magnetic field, opposite in direction to the firstmagnetic field, bends the electron beam into approximate alignment withthe axis of the cathode-ray tube. But since the negative ions areessentially unaffected by the magnetic fields, they continue theirtravel on the ofi-axis deflection path, and eventually strike anelectrode that collects such ions.

Despite the two magnetic fields provided in the ion trap, it is stilldifficult to re-center accurately the deflected electron beam on theaxis of the electron gun. This difficulty remains because it has notbeen possible to produce magnetic fields that are narrow and sharpenough to compensate exactly for the deflection of the electronsintroduced by the tilted electrostatic lens. The two magnetic ion trapfields also tend to distort the electron beam.

An object of this invention is to improve the centering of theelectrostatically deflected electron beam in a cathode-ray tubecontaining an ion trap.

Another object of this invention is to reduce the magnetic distortion ofthe electron beam in the region between the cathode and the tiltedelectrostatic lens in the cathode ray tube.

Another object of this invention is to provide narrow magnetic ion tr-apfields that produce more accurate compensation for the Ideflection ofthe electron beam caused by the tilted electrostatic lens in acathode-ray tube ion trap arrangement.

Anther object of this invention is to provide accurate centering of theelectrostatically deflected electron beam in a cathode-ray tube'containing an ion trap, by means that are inexpensive and simple.

To provide a more accurate and sharply defined magnetic ion trap fieldin the cathode-ray tube ion trap, this invention shields the controlgrid of' the electron gun from the magnetic ion trap field. Thisshielding may be accomplished by placing a ferro-magnetic material, suchas iron, near the control grid, and between the control grid and themagnetic field. This shielding isolates a portion of the beam pathenclosed by the control grid r 2,810,091 Patented Oct. 15, 1957 ICC fromthe magnetic ion trap field, thus effectively narrowing the magneticfield as shown in the following drawings, in which:

Fig. l is a longitudinal sectional view of a cathode ray tube showingthe beam paths in an ion trap not utilizing this invention. v

Fig. 2 is a longitudinal sectional view of a portion of a cathode-raytube showing the beam paths in an ion trap utilizing one embodiment ofthis invention.

Figs. 3, 4, and 5 are longitudinal sectional views of other embodimentsof the magnetic shield.

Referring first to Fig. l, a typical cathode-ray tube is indicated by10. The tube 10 is evacuated and has a glass neck 11 closed at one end,and a diverging glass or metal bulb 12 attached to the opposite end ofthe neck 11. The bulb 12 is closed by a glass face plate 13 that iscoated on its inner surface with a fluorescent screen 15. Theneck 11contains the electrongun structure which, in its usual form, comprisesan electron emitting cathode 16, a heater filament 18 contained withinsaid cathode, a cylindrical control grid 2f) having a centrallyapertured disk 21 at its end toward the glass plate 13, a cylindricalfirst anode 22 having a centrally apertured disk 23 at its end next tothe control grid 2f), a cylindrical second anode 24 having a centrallyapertured disk 25 at its end toward the glass plate 13, and a thirdanode 26 formed by coating the inner surface of a portion of the tubeneck 11 and bulb 12 with an electrically conducting material such asaquadag. This third anode 26 may or may not be electrically connected tothe second anode 24, depending on the exact purpose and use of thecathoderay tube. The control grid 2f) and disk 21, the first anode 22and disk 23, and the second anode 24 and disk 25 are all made of anonmagnetic, metallic material such as stainless steel, and arephysically spaced so that the longitudinal axis of each coincides withthe longitudinal axis 40 of the electron gun. An electromagneticdeflecting yoke 28 encircles the neck 11. ln practice, this yoke 28includes a pair of horizontal deflecting coils and a pair of verticaldeflecting coils. However, since these coils form no essential part ofthis invention per se and yoke 23 is merely indicated.

The cathode-ray tube, as shown in Fig. l and las generally used, has theusual potentials applied to the various components of the electron-gunstructure so as to project the electrons emitted from the cathode towardthe screen Y15. The electrostatic lens 36, formedl between the firstanode 22 and the second anode 24k has its axis of symmetry tilted aboutthe axis 4f) of the cathode-ray tube gun. The two magnetic ion trapfields, which are in the region surrounding the electrostatic lens 3i),and which transversely deflect the beam of electrons in oppositedirections are not shown, but their resultant lines of force `arerepresented by the lines 32 and 34 which are terminated by arrowsindicating the directions of the forces.

In Fig. l, the beam 50 formed by the electron gun contains bothelectrons and negative ions. The electrons of the composite beam aredeflected in the direction indicated by the line of for-ce 32 of onemagnetic ion trap field so that they follow path S2. The ions continuealong their original path 50 until they enter the region of theelectrostatic lens 3f). This lens deflects the negative ions so thatthey follow path 54 and strike the second anode 24 which collects them.This lens 3f) also deflects the beam of electrons so that it followspath 56 back toward the electron-gun axis 40. Under the resultantinfluence of the electrostatic lens 30 and the forces 32 and 34 of themagnetic ion trap fields, the electrons of the beam finally proceedalong path 58.

It is desirable that the last deflected path SS of the electron beamcoincides as nearly as possible with the axis 40 of the cathode-raytube. However, because of the unrestricted magnetic fields previouslyemployed to redeliect the electron beam, it has beendiicult to recenteraccurately the electron beam so that it coincides a nonmagneticAmetallic cylinder and, an apertured disk 21 in accordance with thisinvention lmade of a ferromagnetic material such as iron.` Because ofits high permeability, the ferromagnetic disk '217 shields the spacewithin control grid`20 lfrom magnetic fields, thus narrowing and sharplydefining the magnetic ion trap fields 32 and 34., As a result of thisnarrowing action on the magnetic fields, lthe electron beam isnotdeflected by the magnetic fields 32 until itreaches a point 51 outsidethe shielding effect of the apertureddisk V21. Under the resultantinuenceof Athe electrostatic lens 30 and the forces 32 and 34 ofthemagnetic ion trap fields, the electron beam follows paths S2', 5'6 andfinally 58. As before, the ions follow path 54 and are collected bythesecond anode 24. Not only does the ferromagnetic disk 21' cause path 58to coincide more closely with the cathode-'ray tube axis 40, butit'rednces the distortion of the electron beam. vThese advantages areaccomplished because the shielding action of the disk 21" restricts themagnetic fields, thus allowing them to compensate more precisely for theelectrostatic deflection without distorting the electron beam any morethan necessary.

Another embodiment of 4this invention is shown in Fig. 3, wherein onlythe control grid portion of the electron-gun structure is shown. Insteadof the control grid 20 and disk 21 both being made of a nonmagneticmetallic material, the cylinder 20 is made of a ferromagnetic materialsuch as iron. This magnetic material shields the control grid in thesame manner as did the magnetic apertured disk 21 in Fig.'2, therebyre-centering the electron beam more accurately and introducing lessdistortion to it.

Figs. 4A and 4B show `-diierent constructions of another embodiment 'ofthis invention. Instead of replacing the usual nonmagnetic, metallic,apcrtured disk 2'1 in Fig. 4A, or external of the control grid, as shownin r Fig. 4B. In either case, it shields the control grid in the same`way as did the ferromagnetic `apertured disk 21' shown in Fig. 2,`togive the same advantages.

Figs. 5A and 5B show different forms of another embodiment of theinvention. Instead of replacing the usual nonmagnctic metallic cylinder20 with a ferromagnetic cylinder 20 as shown in Fig. 3, a ferromagneticcollar 20 is positioned adjacent to the cylindrical portion 2G of thecontrol grid either outside the cylindrical portion, as shown in Fig.5A,"or within the cylindrical portion, as shown in Fig. 5B. Theferromagnetic collar 20 also shields the control grid 20 from themagnetic fields as did the ferromagnetic `cylindrical grid as shown inFig. 3, thus restricting the magnetic field to give more accurate beamre-centering with less beam distortion.

What is claimed is:

1. An electron gun for a cathode ray tube adapted for use with anexternal magnetic field, said gun comprising an electron source and aplurality of electrodes spaced therefrom for forming an electron beamalong a path, one of said electrodes including a wall portion forshielding said beam path from said magnetic field.

2. An electron gun for a cathode ,ray tube adapted for use with anexternal magnetic field, said gun comprising an electron source, acontrol electrode insulatingly spaced from said electron source, and anadditional electrode insulatingly spaced from said control electrode toform electrons from said source into -a beam, said control electrodeincluding a ferromagnetic wall portion for shielding said electronsource from said magnetic field.

3. The Vinvention of claim 2, wherein said ferromagnetic wall portionincludes an apertured member adjacent to said Velectron source.

4. The invention .of claim 2, wherein said ferromag netic wall portionincludes a tubular wall portion enclosing said electron source.

5. `An electron Vgun for-a cathoderay tube adapted for use with anAexternal 4magnetic field, said Ygun comprising an electron source, acontrol electrode insulatingly spaced from said electron source, and anadditional electrode insulatingly spaced from said .control electrode toform electrons from said source into a beam, and a ferro magnetic wallkportion surrounding said electron source for magnetically shieldingsaid source from said magnetic field.

6. An electron gun for a cathode ray tube adapted for use with anexternal magnetic field, said gun comprising an electronsource, acontrol electrode insulatingly spaced from said electron source, and anadditional electrode insulatingly spaced from said control electrode toform electrons from said source into a beam along a path, and anapertured ferromagnetic wall portion spaced from said electron sourcealong said beam path for shielding said source from said magnetic field.

References Citedin the le of this patent UNITED 'STATES PATENTS2,522,872. Heppner Sept. 19, 1950

